ALSA: seq: Define driver object in each driver

ALSA: seq: Define driver object in each driver

This patch moves the driver object initialization and allocation to
each driver's module init/exit code like other normal drivers.  The
snd_seq_driver struct is now published in seq_device.h, and each
driver is responsible to define it with proper driver attributes
(name, probe and remove) with snd_seq_driver specific attributes as id
and argsize fields.  The helper functions snd_seq_driver_register(),
snd_seq_driver_unregister() and module_snd_seq_driver() are used for
simplifying codes.

Signed-off-by: Takashi Iwai <tiwai@suse.de>